Detection and measurement of radiation (photons and particles) are critical to many scientific and industrial applications. Although the present invention can be used for radiation detection in general, the following discussion relates primarily to x-ray detection.
The measurement of the energy of radiation is particularly of interest for many scientific and industrial applications, such as x-ray fluorescence (XRF) microanalysis. XRF microanalysis can be used for elemental and chemical analysis. For example XRF microanalysis is used for the detection and spatial imaging of defects in semiconductor materials. XRF microanalysis can be performed using a range of sources, including electron beams (e.g., scanning electron microscopes), x-ray beams (e.g., synchrotron or x-ray generators), and ion beams (e.g., particle induced X-ray emission (PIXE)).
The most commonly used detectors for XRF microanalysis are semiconductor-based (e.g., Silicon and Germanium). The energy resolution of current state-of-the-art x-ray detectors based on Si or Ge have reached their fundamental energy resolutions of ˜100 eV at 6 keV. The energy resolution of these detectors is limited by the counting statistics (Fano noise limit) implied by the large size of the semiconductor energy gap (i.e., the electron-hole pair excitation energy, a few electron volts). This resolution severely limits the range of information that can be gained from XRF microanalysis. For example, XRF microanalysis is used to map the two-dimensional distribution of metals in modern integrated circuit (IC) chips. However, these modern IC chips use heavy metals (e.g., Er, Hf, Ta, Tb, Tm), in addition to the typical metals (e.g., Ti, Cu, Mo, Pd, Ag, In). The L-edge fluorescence lines of these heavy metals overlap with the K-edge fluorescence lines of the typical metals. Thus, detectors with an energy resolution <50 eV are desirable for XRF microanalysis of modern integrated circuits.
As an alternative to semiconductor-based detectors, superconductors have energy gaps that are roughly three orders of magnitude smaller than semiconductors. In a device called a microwave kinetic inductance detector (MKID) a superconducting thin film is patterned into an electrical resonator (Day, P. K., LeDuc, H. G., Mazin, B. A. Vayonakis, A. & Zmuidzinas, J. “A broadband superconducting detector suitable for use in large arrays”. Nature 425, 817-821 (2003)). The operating temperature of these detectors is below ˜1 K. The inductance of the superconductor varies as a function of the number of quasiparticles (charge carriers). When the superconductor absorbs a photon, Cooper pairs break apart creating excess quasiparticles. The number of Cooper pairs that break (and number of excess quasiparticles created) is a function of photon energy and the superconductor energy gap. This detection mechanism is non-equilibrium or athermal (i.e., the detector temperature does not change). The increase in quasiparticles alters the inductance of the resonator and thus the resonance frequency. This change in resonance frequency can be measured as the change in the phase and amplitude of a microwave signal transmitted past the resonator and can be correlated to the photon energy. While these devices typically operate more slowly than semiconductor devices, they are naturally frequency-multiplexable by designing each pixel (superconducting resonator) to have a different resonance frequency and coupling the resonators to a single microwave feed line. The ability to multiplex is a key design consideration for very low temperature detector systems. The limited cooling capacity at very low temperatures calls for limiting the number of wires or cables from room temperature to the detector. The ability to readout multiple pixels on a single wire or cable is highly desirable for very low temperature detectors; this is often referred to as the multiplexing factor. Many applications require large pixel counts for imaging or count rate requirements. MKID arrays of approximately 2000 pixels have been deployed and arrays with 10,000 pixels are being designed today.
An alternate method for x-ray detection is a microcalorimeter. A microcalorimeter does not count charge carriers, but measures a change in temperature. A microcalorimeter consists of an absorber with a weak thermal link to a heat bath and a sensitive thermometer. When a photon is absorbed, the temperature of the absorber increases by an amount determined by the photon energy and the absorber heat capacity. The thermometer measures the temperature rise. The ultimate resolution of such a device is determined by the sensitivity of the thermometer and the thermal fluctuations over the weak thermal link, not counting statistics. The energy resolution increases with decreasing operating temperature. One such device is a superconducting transition edge sensor, which relies on the sharp transition with temperature between the superconducting and normal state (USPTO U.S. Pat. No. 5,880,468).
MKIDs suffer from the same Fano noise limits as the semiconductor devices. While their resolution will be better than semiconductor devices due to the lower energy gap, for applications with x-ray photons they suffer from other issues such as background signal due to substrate absorption and low stopping power, which limits overall efficiency.
While superconducting microcalorimeters (e.g., using transition edge sensors as the thermometer) have achieved good energy resolution (˜1 eV), they employ complex low temperature electronics (SQUIDs) that limit the number of pixels in the detector and thus overall solid angle and total count rate.